Horological hairspring



Sept. 15, 1970 .JLG. SUARD ,237

HOROLOG ICAL HAIRSPRING Filed Apri1'30, 1968 A (PRIOR ART) INVENTOR. JEAN 6. SMRO A T TQRMEYS United States Patent 3,528,237 HOROLOGICAL HAIRSPRING Jean G. Suard, Besancon, France, assignor to Timex Corporation, Waterbury, Cnn., a corporation of Delaware- Filed Apr. 30, 1968, Ser. No. 725,275 Int. Cl. G04c 3/04 U.S. Cl. 58-23 2 Claims ABSTRACT OF THE DISCLOSURE A horological movement includes a balance wheel fixed to a balance wheel staff. A hairspring, attached between the balance wheel assembly and the frame, permits oscillation of the balance wheel. The hairspring is formed in an irregular convolute, i.e., with a non-uniform pitch, and is round in cross-section.

The present invention relates to horology and more particularly to the spiral hairspring and balance wheel assembly of a watch.

A horological instrument requires some device to provide a regular beat. In a standing clock the beat may be provided by a swinging pendulum. Watches for many years have used a circular balance wheel for that function. The balance wheel is fixed to a staff which is free to pivot in bearings or jewels. The balance wheel is oscillated by means of a spiral hairspring and an impulsing means. The hairspring is usually wound in a number of turns and formed from spring steel having a rectangular crosssection. Mechanical watches use a single hairspring. Electrical watches also use a single hairspring which, when the coil is carried by balance, carries electrical current to the cor A hairspring in a watch is critical to its timekeeping. If the spring force of the hairspring changes, for example, with temperature variations, the passage of time and the inlilfuence of magnetism, the accuracy of the watch may su er.

The usual watch hairspring, as a result of research efforts over many years, is often constructed of the high quality and expensive steel alloys. The cleaning and heat treatment of the hairspring is carefully controlled. The hair spring is, in practice, exactly wound so that its pitch, i.e., degree of curvature, is constant, following an Archimedes spiral formula. This type of hairspring winding presents a problem in that the outer turns may be close enough together to clash against each other during the movement of the balance wheel which winds up the hairspring. Such clashing causes a loss of accuracy.

The height of the hairspring, i.e., its dimension parallel to the axis of the balance wheel staff, may present a problem. The height of the hairspring adds to the thickness of the watch, particularly if the watch is an electronic watch which uses two hairsprings.

It is the objective of the present invention to provide a hairspring and balance wheel assembly whose hairspring has a relatively low height, which has a relatively large number of turns in a given space without clashing of the outer turns, which may be less subject to adverse air and magnetic effects, and which may be inexpensive to manufacture.

In accordance with the present invention, a balance wheel and hairspring assembly is provided for a Watch. The hairspring is wound with a certain formula in which its curve is not a simple function, that is, its pitch is not constant. The hairspring wire, in cross-section, is round.

The hairspring is wound with its turns toward the center more tightly wound than its outer turns, which avoids clashing of the outer turns. The round cross-section of the hairspring requires less vertical space, i.e., parallel to the axis of the balance wheel staff. The saving in vertical space may be of particular importance in electronic watches using two hairsprings arranged in different parallel planes. A hairspring which is round in cross-section, compared to one having a rectangular section of the same force and length, would Weigh less. The lighter spring, of round cross-section, will sag less when the watch is in its normal position, i.e., normal relative to the balance wheel axis. This will result in improved timekeeping, due to less position error, as there will be less harmful effect on timekeeping in the vertical position.

The sagging which may occur in the horizontal position of the watch will not affect the timekeeping of the watch. In addition, the hairspring having a round cross-section may be comparatively less affected by stray magnetic flux and by air pressure changes. The round hairspring may be formed from relatively inexpensive and round drawn wire.

The hairspring having a round cross-section, if wound with the conventional constant pitch, would require a larger outside diameter than is attainable by using a nonconstant pitch.

Other objectives of the present invention will be apparent from a description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:

FIG. 1A is a top plan view of a prior art watch hairspring;

FIG. 1B is a cross-section along lines AA of the hairspring of FIG. 1A;

FIG. 2A is a top plan view of the hairspring of the present invention;

FIG. 2B is a cross-section along line B--B of the hairspring of FIG. 2A; and

FIG. 3 is a perspective view of the hairspring of FIGS. 2A and 2B connected in a balance wheel assembly.

The conventional prior art watch hairspring of FIGS. 1A and 1B consists of a strip of spring steel whose crosssection is rectangular and uniform along its length. The height of the spring is the long part of the rectangular cross-section parallel to the axis of the balance wheel staff. The conventional spring, having a constant pitch, is wound according to the following formula:

=A +Ka where:

=coordinate of a point of the hairsprings;

A=a constant giving the inside radius of the hairspring;

K=a constant providing the pitch, i.e., degree of curvature, of the hairspring:

6=the angular coordinate If a balance wheel staff has a hub requiring that the spring have a certain radius A, the above formula then becomes simply p=K0, which is an Archimedes curve.

The Archimedes curve has a long history, particularly in the later Middle Ages, as possessing almost magical properties. On a more practical basis, the Archimedes curvehas been considered relatively simple to measure to a selected tolerance and relatively simple to manufacture.

The rectangular cross-section has been utilized as it presented a stilfness against the drooping of the hairspring, i.e., deflection parallel to the axis of the balance wheel staif. However, the rectangular cross-section, when compared to a hairspring of round section, requires a larger cross-section to obtain the same spring force. In other words, in a given space, more force may be obtained from a spring of round cross-section than a spring of rectangular cross-section. With a spring of rectangular cross-section, the moment of elasticity (return torque) Mr is where E is the modulus (coeoflicient) of elesticity, w is the width and H the height of the cross-section. A Spring of round cross-section has its moment of elasticity where E is the modulus of elesticity, and r is the radius of the cross-section. One may compare a spring having a round cross-section with one having a rectangular crosssection; and, using the following assumptions, it is found that a rectangular spring, delivering the same force as a round spring, has a cross-sectional area Ar which is more than twice as large as the cross-sectional area Ad of a round spring, i.e., Ar:Ad=1.00:0.46. The assumptions made are (1) the same material is used, so that E is the same in both types of springs; (2) both springs are wound with the same pitch; (3) the inside radius A of both springs is the same; (4) the lengths l of both springs are equal; (5) the moments of elasticity of both springs are equal, i.e., Md=Mr; (6) the ratio, in the spring having a rectangular cross-section, of its width w to its height is 1 to '5, a conventional ratio for such springs.

As a specific example leading to the relationship Ar:Ad=1.00:0.46, typical dimensions of a rectangular shaped spring as, for instance, in a wrist watch, having the conventional ratio, Width w to height h, of l to 5, are width of .03 mm. and height of .15 mm. The assumption is made that Md=Mr, therefore:

121 41 Since modulus of elasticity E, angular constant a and length l are assumed to be equal for both the rectangular spring and the round spring, expression (1) above becomes:

12 4 where;

3 (3) Mra and (4) .Zllda Using w=.03 mm., h =.1 5 mm. in expression (3) above: Mm g :33.8X 10- Since Md=Mr, then:

Solving expression for r:

r=2.56 10' =.0256 mm.

Cross-sectional area rectangular spring:

Ar=w'h=.03 (.15 =.0045 =45 1-0- mm.

Cross-sectional area round spring:

Ad=1rr =1r(.()25'6) =20.6 10 mm.

Relationship of Ar to Ad:

Ar:Ad=45 X 10- :20.6' 10- Therefore, Ar'zAdl: 1.00: .46

The regular (Archimedes) curve, i.e., a constant pitch,

has been utilized in conventional hairsprings as it was 4 widely thought to provide an even spring power and because it is relatively easy to wind. However, a spring may be wound according to a preselected curvature, i.e., a variable pitch, according to the following formula, in accordance with the present invention:

=coordinate of a point of the hairspring A=a constant giving the inside radius of the hairspring K=the pitch of the hairspring 0=the angular coordinate F=function b, c =constants The function F (0, b, c is selected to obtain one or more of the requirements of the hairspring, such requirements including the pitch determined by the theoretical equations of chronometry, the result of experimental testing, the characteristics of the spring material, the heat treatment to be carried out in the spring and on the wire from which the spring is formed, the mechanical working of the wire, and other processing steps.

The function G[0, i, k 1 provides the pitch for the outside coil turn of the hairspring coil. The outside turn is selected to match the regulator so that the rate may be set.

Preferably the interior convolutions (windings) of the hairspring 10, as shown in FIG. 2, are at a relatively lesser pitch than the outer windings. The clashing of the outer windings is avoided by spacing them further apart.

As an example of the hairspring according to the present invention, a hairspring having twelve coils (complete turns) is wound with a pitch of two thicknesses of the hairspring starting at the first coil, to a final pitch of 3 or 4 or 5, the pitch continuously growing from 2 to 3 or 4 or !5. This type of hairspring may, compared to hairsprings having a constant pitch, be of greater length within a given volume or, alternatively, a given length of hairspring would have a smaller outside diameter.

The hairspring of the present invention, having a nonregular pitch and a round cross-section, finds particular application in an electronic watch. In the electronic watch shown in FIG. 3 a balance wheel 20 is mounted on a balance wheel staff 21. The staif rotates freely in jewels or other bearings in the frame plate 22 and the bridge 23. The balance wheel carries a round coil 24 having two terminal wires 26 and 26a. The coil 24 interacts with a series of three north-south-north permanent magnets 25 fixed to the shunt plate 25a. A circuit 27 having terminals 30 and 31 is connected to a battery (not shown). The circuit 27 pulses the coil 24 with current during its passage through the magnetic fields of magnets 25. A suitable detailed circuit and magnet structure is explained in Zemlas U.S. Pat. 3,046,460, issued July 24, 1962.

The coil terminals 26 and 26a are connected to their respective circuit terminals 30 and 31 through the hairsprings 32 and 33. The hairsprings are round in crosssection and are wound with a non-regular pitch. One hairspring 32 is connected to terminal 26 of the coil and is on one side of the balance wheel, and the other hairspring 33 is connected to terminal 26a on the other side of the balance wheel. The hairspring outer ends are fixed in insulated posts 34, 35 in the frame and bridge. Their other ends are fixed in hairspring hubs on the balance wheel stafl.

A drive 36 fixed to the balance wheel interacts with the arms of a fork lever 37 to oscillate the lever 37. The lever 37 is pivoted on staif 38 and has banking pins (not shown) which limit its movement. A pin 39 on the opposite end of the lever engages in the teeth of ratchet (index) wheel 40 and indexes the wheel. The movement of wheel 40 is transmitted by a conventional gear train to the hands of the watch.

The hairspring of the present invention is also adapted for use in a mechanical watch, which is powered by a mainspring. The mainspring is housed in a barrel having exterior gear teeth and is wound by a crown, through one or more gears. The barrel meshes with a set of gears to drive the time indicating hands. The release of the mainspring power is controlled by an escapement which includes a lever and a balance wheel assembly, the lever being pivoted and providing an impulse to the balance wheel. The balance wheel assembly pivots between the movement frame and a bridge. A hairspring, of round cross-section and non-uniform in its Winding pitch, is connected between the bridge and a hub on the balance wheel staff.

Modifications may be made in the present invention within the scope of the subjoined claims.

I claim:

1. An electronic watch including a frame, a bridge connected to the frame, a balance wheel assembly rotatably positioned between said frame and said bridge, an electronic circuit including means to be connected to a battery, a coil carried on the balance wheel, means to create at least one magnetic field positioned to be interacted with the coil, a first hairspring connected between the frame and the balance wheel assembly, and a second hairspring connected between the bridge and the balance wheel assembly, wherein the first and second hairsprings each are electrical connections between the circuit and the coil, and wherein at least one of the hairsprings has a plurality of its windings in a common plane, is round in cross-section and non-uniform in winding pitch.

2. An electronic watch as in claim 1 wherein both hairsprings have a plurality of windings in a common plane, are round in cross-section and non-uniform in winding pitch.

References Cited UNITED STATES PATENTS 2,196,866 4/1940 Juillerat 58115 3,186,157 6/1965 Favret et al. 58-107 FOREIGN PATENTS 1,217,857 5/1960 France.

RICHARD B. WILKINSON, Primary Examiner E. C. SIMMONS, Assistant Examiner US. Cl. X.R. 58107, 114 

